Fuel efficiency and emission generation characteristics are important concerns in the design and operation of an internal combustion engine. An internal combustion engine may be used in a vehicle, such as, for example, an on or off highway truck, a work machine, or an automobile. In addition, an internal combustion engine may be used in a stationary system, such as, for example, a power generator.
An internal combustion engine of a vehicle may be operated to generate the power required to both propel the vehicle and to operate any auxiliary systems associated with the vehicle. The vehicle may include a transmission that connects the internal combustion engine with a ground engaging device, such as, for example, a set of wheels or tracks. In addition, the vehicle may include a “power take off” unit that is connected to the internal combustion engine and that directs power from the internal combustion engine to operate any auxiliary devices associated with the vehicle.
An internal combustion engine may be tuned to achieve optimal fuel efficiency under certain operating conditions, such as when operating at a certain engine speed and load. Typically, an engine is designed and tuned to achieve optimal fuel efficiency when the vehicle is operating in steady-state conditions such as, for example, at an expected cruising speed. When the vehicle is operated outside of these steady-state conditions, such as, for example, when the vehicle is accelerating, decelerating, or idling, the vehicle may experience increased losses that result in a reduction in the fuel efficiency of the engine.
A vehicle may use an alternative power source to compensate for these types of efficiency losses. As shown in U.S. Pat. No. 5,350,031 to Sugiyama, a vehicle may include an electric drive that is disposed between the engine and the ground engaging devices. The electric drive may include a generator, an electric motor, and a storage device, such as a battery. Operation of the engine may drive the generator, which provides power to the electric motor to thereby drive the ground engaging devices. Excess power that is generated, but is not used to drive ground engaging devices or power any auxiliary systems, may be directed to the storage device.
The internal combustion engine and generator may generate excess power under several different operating conditions, such as, for example, when the vehicle is decelerating or idling. During these circumstances, the excess power may be saved in the storage device for later use. The stored power may be used, for example, when the vehicle requires an increased amount of power, such as when the vehicle is accelerating. In this manner, the energy storing capabilities of the electric drive may be used to reduce the magnitude of the loses associated with both vehicle deceleration and acceleration.
However, in this type of electric drive system, the internal combustion engine may be operated outside of its optimal operating conditions. For example, when the vehicle is accelerating, the engine speed and load be increased above the optimal operating conditions of the engine. In addition, when the vehicle is idling, the engine speed and load may be below the optimal operating conditions of the engine. Accordingly, an internal combustion engine used with this type of electric drive system may still experience the reduction in fuel efficiency associated with operating the engine outside of the optimal operating conditions.
The disclosed electric drive system solves one or more of the problems set forth above.